The field of the disclosure relates generally communication networks, and more particularly, to communication networks implementing full duplex transmission.
A duplex communication system is a point-to-point (P2P) system composed of two connected parties or devices that can communicate with one another in both directions. Generally, a duplex system has two clearly defined data transmission channels, with each channel carrying information in one direction, i.e., upstream or downstream. In a full duplex system, both parties/devices can communicate with each other simultaneously using the same physical spectrum channels. That is, information travels simultaneously in both the upstream and downstream directions of a single transmission medium (e.g., fiber optic, cable, etc.).
In a cable network that transmits radio frequency (RF) signals, full duplex communication has been difficult to achieve due to the number of downstream devices, as well as the network configuration. For example, a modem termination system (MTS, or a cable MTS (CMTS)) is typically coupled to a plurality of downstream modems (including cable modems (CMs)). Each of the modems transmits upstream along the same path to the MTS, while the MTS transmits its downstream communications to the modem(s). The modems and the MTS though, generally utilize different spectral bands from one another, and the modems may transmit at specific coordinated times.
In typical operation, however, many modems transmit to the MTS at the same time, and thus tend to interfere with one another, and also with the MTS if they are not separated in time and spectrum bands, as required by conventional Frequency Division Duplex (FDD), Time Division Multiple Access (TDMA), and Time Division Duplex (TDD) techniques. Full duplex has been implemented in access networks utilizing hybrid fiber-coaxial (HFC) and/or radio frequency over glass (RFoG) architectures, but is not limited to only these types of communication systems.
Systems and methods for full duplex transmission are illustrated and described in greater detail in U.S. Pat. No. 9,762,377, which is incorporated by reference herein. As described in this earlier patent, full duplex is accomplished for a communication system in an RF cable network utilizing a CMTS of a headend/hub in operable communication with a plurality of CMs (e.g., through a node) over a duplex communication link, which may include a fiber optic transmission medium, and implementing a communication protocol such as Data Over Cable Service Interface Specification (DOCSIS). Under these configurations, the CMTS is operable to send control signals that direct the CMs to operate in a particular manner with respect to the employed cable protocol.
Amplification of full duplex transmissions, however, has been challenging. Conventional amplifiers used in HFC networks, for example, are designed to amplify distinct non-overlapping portions of the transmitted spectrum in either direction. In one example, a mid-split amplifier amplifies a 5-85 MHz spectral portion in the upstream direction, and a 108-1218 MHz spectral portion in the downstream direction. However, a full duplex should be capable a full duplex portion of the spectrum (e.g., 108-684 MHz) in both directions. One particular challenge associated with amplifying this common spectrum in both directions is the susceptibility of the amplifier to the ringing effect or amplifier instability. Instability resulting from a bleed-over effect between the downstream and upstream transmissions is described further below with respect to FIGS. 1-3.
FIG. 1 is a schematic illustration of an apportioned frequency spectrum 100 for dual-direction transmission in a communication system. Frequency spectrum 100 includes a first spectral portion 102 designated for signals transmitted in the downstream direction (e.g., from the hub/headend MTS to one or more modems), and a second spectral portion 104 designated for signals transmitted in the upstream direction (e.g., from a modem to the MTS). First spectral portion 102 is a separated from second spectral portion 104 about a center frequency fc between the respective portions. In this example, first spectral portion 102 is depicted as spanning the range between 85 MHz and 1002 MHz, whereas second spectral portion 104 is depicted as spanning the range between 5 MHz and 75 MHz. These spectral ranges though, are provided for illustrative purposes, and are not intended to be limiting. That is, different spectral ranges may be utilized in the upstream and downstream directions without departing from the scope of the systems and methods described herein.
FIG. 2 is a schematic illustration depicting a sequential amplifier bleed-over effect 200 from a downstream transmission 202 onto an upstream transmission 204 using an amplification subsystem 206 in a conventional duplex communication system. Amplification subsystem 206 includes a downstream amplifier 208, an upstream amplifier 210, a first splitter/combiner 212 (e.g., disposed between amplification subsystem 206 and the MTS), and a second splitter/combiner 214 (e.g., disposed between amplification subsystem 206 and one or more modems).
In operation (i.e., sequence steps ID-VID), downstream transmission 202 is initially received from the MTS by first splitter/combiner 212, and is substantially confined within first spectral portion 102D(I). For ease of explanation, the MTS is presumed, for purposes of this discussion, to transmit no downstream signal within second spectral portion 104. That is, at sequence step ID, second spectral portion 104(ID) is a substantially equal to zero. As described below with respect to FIG. 3, however, some portion of upstream transmission 204 will be seen (i.e., bleed over) at first splitter/combiner 212. Accordingly, even though both downstream transmission 202 and upstream transmission 204 affect each other, the description of the embodiments of FIGS. 2 and 3 addresses each respective effect individually, for ease of explanation.
In further operation of amplification subsystem 206, at sequence step IID, downstream transmission 202 is amplified by downstream amplifier 208, as symbolically indicated by the increased amplitude of first spectral portion 102(IID) (i.e., after passing through downstream amplifier 208) with respect to first spectral portion 102(ID). In this example, downstream amplifier 208 is depicted as having a 25 dB gain, but this gain value is provided merely for purposes of illustration, and not in a limiting sense. Thus, ideally, second splitter/combiner 214 would ideally receive and transmit to one or more modems a “clean” amplified downstream transmission 202 conforming to the illustrated shape of first spectral portion 102(IID).
However, at sequence step IIID, some of downstream transmission 202 will bleed over into upstream transmission 204 proximate second splitter/combiner 214, as indicated by first spectral portion 102(IIID). At the same time, at sequence step IVD, second splitter/combiner 214 receives upstream transmission 204 from one or more modems second substantially confined within and second spectral portion 104(IVD). Accordingly, as seen at the input to upstream amplifier 210, the spectral representation of upstream transmission 204 at sequence step VD resembles the combination of the spectral representation of upstream transmission 204 at sequence step IVD (i.e., second spectral portion 104(IVD)) and the spectral representation of the bleed-over of downstream transmission 202 at sequence step IIID (i.e., first spectral portion 102(IIID)). At sequence step VID, the entirety of the input spectrum (i.e., first spectral portion 102(VD) and second spectral portion 104(VD)) at sequence step VD is amplified by upstream amplifier 210. Accordingly, at sequence step VID, the MTS will receive a substantially amplified, but undesirable, second spectral portion 104(VID).
FIG. 3 is a schematic illustration depicting a sequential amplifier bleed-over effect 300 from upstream transmission 204, FIG. 2, onto downstream transmission 202 using amplification subsystem 206. Bleed-over effect 300 is similar to bleed-over effect 200, except that bleed-over effect 300 is individually described with respect to sequence steps IU-VIU) that illustrate the respective effect on downstream transmission 202 from upstream transmission 204. The person of ordinary skill in the art will appreciate that both of bleed-over effects 200, 300 may occur simultaneously, and even cause a magnification of the respective effect over time.
In operation of amplification subsystem 206 (i.e., from the upstream perspective), at sequence step IU, upstream amplifier 210 receives a “clean” (for purposes of this discussion) upstream transmission 204 substantially confined within second spectral portion 104(IU). At sequence step IIU, upstream amplifier 210 amplifies (e.g., a 25 dB gain, in this example) upstream transmission 204 to produce an output having the profile of second spectral portion 104(IIU), which spans the same frequency range as second spectral portion 104(IU), but has a greater amplitude.
Similar to bleed-over effect 200, at sequence step IIIU, some of upstream transmission 204 will bleed over into downstream transmission 202 proximate first splitter/combiner 212, as indicated by second spectral portion 104(IIIU). At the same time, at sequence step IVU, first splitter/combiner 212 receives downstream transmission 202 from the MTS, which is substantially confined within second spectral portion 102(IVU). Accordingly, as seen at the input to downstream amplifier 208, the spectral representation of downstream transmission 202 at sequence step VU resembles the combination of the spectral representation of downstream transmission 202 at sequence step IVU (i.e., first spectral portion 102(IVU)) and the spectral representation of the bleed-over of upstream transmission 204 at sequence step IIIU (i.e., second spectral portion 104(IIIU)). At sequence step VIU, the entirety of the input spectrum (i.e., first spectral portion 102(VU) and second spectral portion 104(VU)) at sequence step VU is amplified by downstream amplifier 208. Accordingly, at sequence step VIU, the modem(s) will receive a substantially amplified, but undesirable, first spectral portion 102(VIU).
The person of ordinary skill in the art will thus understand how, in conventional configurations, the undesirable bleed-over portions can continue to cycle back and forth between downstream transmission 202 and upstream transmission 204 and be continually amplified in each cycle and destabilize the respective transmissions thereby. Accordingly, it is desirable to provide an amplifier system or technique for full duplex transmissions that avoids the instability, bleed-over, and ringing problems that are conventionally encountered.